Purity, Formulations and Chromatography

This lesson covers key concepts in chemical analysis from the AQA GCSE Combined Science Trilogy (8464) specification. You need to understand the scientific meaning of purity, what formulations are, and how chromatography works to separate and identify substances.

---

Purity in Chemistry

In everyday language, "pure" means "not mixed with anything harmful." In chemistry, pure has a very specific meaning:

Key Definition: A pure substance is a single element or compound, not mixed with any other substance.

Melting and Boiling Points of Pure Substances

A pure substance has a sharp, fixed melting point and boiling point. An impure substance (mixture) melts and boils over a range of temperatures.

Property	Pure Substance	Impure Substance (Mixture)
Melting point	Sharp and fixed	Melts over a range; lower than expected
Boiling point	Sharp and fixed	Boils over a range; higher than expected

Example: Pure water boils at exactly 100°C and freezes at exactly 0°C. Adding salt to water raises the boiling point and lowers the freezing point.

Exam Tip: If a question gives you melting point data and asks whether a substance is pure, check if the melting occurs at a single temperature (pure) or over a range (impure).

---

Formulations

A formulation is a mixture that has been designed as a useful product in which every component is present in a measured quantity and has a particular purpose.

Examples of Formulations

Formulation	Components and Purposes
Paint	Pigment (colour), solvent (dissolves binder), binder (sticks to surface), additives (drying, texture)
Medicine (tablet)	Active ingredient (drug), binder (holds tablet together), filler, coating
Alloy (e.g. steel)	Iron + carbon + other metals in precise proportions
Fertiliser	Nitrogen, phosphorus, potassium compounds in known ratios
Cleaning product	Surfactant, solvent, fragrance, water in precise amounts

Key Point: A formulation is not just any mixture — each component is present in a precise, measured quantity for a specific purpose. This is what distinguishes a formulation from a random mixture.

---

Chromatography

Chromatography is a technique used to separate mixtures of dissolved substances (solutes). It can be used to identify unknown substances by comparing them with known substances.

How Paper Chromatography Works

1. Draw a pencil line (baseline) near the bottom of the chromatography paper (pencil, not pen, because pencil is insoluble and will not dissolve).
2. Place spots of the substances to be tested on the baseline.
3. Place the paper in a beaker containing a shallow layer of solvent (e.g. water or ethanol). The solvent level must be below the baseline.
4. The solvent rises up the paper by capillary action.
5. Different substances in the mixture are carried different distances up the paper because they have different solubilities in the solvent and different attractions to the paper.
6. Remove the paper when the solvent is near the top and mark the solvent front.

flowchart TD
    A["Draw pencil baseline near bottom of paper"] --> B["Spot samples on baseline"]
    B --> C["Place paper in solvent — solvent below baseline"]
    C --> D["Solvent rises up paper by capillary action"]
    D --> E["Substances carried different distances"]
    E --> F["Remove paper and mark solvent front"]
    F --> G["Calculate Rf values"]

Interpreting a Chromatogram

Observation	Meaning
Only one spot	The substance is a single, pure compound
Multiple spots	The substance is a mixture
Spots at same height	The substances are the same (or very similar)
Spots at different heights	The substances are different

---

Calculating Rf Values

The Rf value (retention factor) is used to identify substances. It is calculated as:

$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$Rf​=distance moved by solvent frontdistance moved by substance​

Both distances are measured from the baseline (not from the bottom of the paper).

Worked Example

A substance moves 3.6 cm from the baseline. The solvent front moves 9.0 cm from the baseline.

$R_f = \frac{3.6}{9.0} = 0.40$Rf​=9.03.6​=0.40

Properties of Rf Values

Property	Detail
Range	Always between 0 and 1
Unique	Each substance has a characteristic Rf in a given solvent
Depends on solvent	The same substance may have a different Rf in a different solvent
Comparison	Unknown substances can be identified by comparing their Rf with known values

Exam Tip: Always measure from the centre of a spot to the baseline, and from the solvent front to the baseline. Rf values have no units. If your calculation gives a value greater than 1, you have made an error.

---

Common Mistakes

• Using pen instead of pencil for the baseline (pen ink dissolves and runs with the solvent).
• Measuring from the bottom of the paper rather than from the baseline.
• Forgetting that Rf values depend on the solvent used — you must state the solvent.
• Confusing "pure" in chemistry (single substance) with "pure" in everyday language (clean/healthy).

---

Summary

• A pure substance has a sharp, fixed melting and boiling point; impurities cause a range
• A formulation is a designed mixture with components in measured quantities for specific purposes
• Chromatography separates dissolved substances based on their solubility and attraction to the paper
• Rf = distance moved by substance ÷ distance moved by solvent front
• Rf values are used to identify unknown substances by comparison with known values
• A single spot on a chromatogram indicates a pure substance; multiple spots indicate a mixture

Exam Tip: Chromatography questions are common on AQA 8464. Practise calculating Rf values, interpreting chromatograms, and describing the method clearly.

---

Extended Worked Examples and Exam Technique

Worked Example 1: Calculating an Rf Value

Question: In a paper chromatogram, the solvent front moved 9.0 cm from the baseline. A spot of dye moved 6.3 cm from the baseline. Calculate the Rf value.

Solution: $R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}} = \frac{6.3}{9.0} = 0.70$Rf​=distance moved by solventdistance moved by substance​=9.06.3​=0.70

Rf values have no units — they are ratios. They are always between 0 and 1.

Worked Example 2: Identifying an Unknown Dye